System and method for eliminating the structure and edge roughness produced during laser ablation of a material

ABSTRACT

The present technology relates generally to laser ablation, and more particularly pertains to a system and method for eliminating structure and edge roughness, which is produced during the laser ablation of a material. Ablation of materials using a femtosecond laser beam produces a fine scale periodic structure in the ablated region. The structure consists of residual (i.e. unablated material) and is always perpendicular to the polarization direction of the laser beam. By changing the polarization direction during the ablation process, the structure is averaged over many directions and thus eliminated. This eliminates structure and edge roughness in a material caused by the laser ablation of the material. The method is employed to the repairing of photomasks so as to cause the optical quality thereof to be improved.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. Ser. No.11/624,257, filed Jan. 18, 2007.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to laser ablation, and moreparticularly pertains to a system and method for eliminating structureand edge roughness, which is produced during the laser ablation of amaterial.

Typically, in this particular technology, an ultrashort pulsed laserbeam is utilized to ablate undesired extra material, which is present ina photomask. The pulsed laser beam is fired in a programmed spatialpattern, thereby removing the encountered extra material which causes adefect. However, it is possible that the process of laser ablation canproduce a periodic structure in the irradiated material, whereby thisperiodic structure produces a significant roughness at the edge of theablated defect, which degrades the optical quality of the repairedphotomask. Consequently, there is a need to provide a system and methodwhich will prevent or eliminate this edge roughness, and thereby toresultingly produce a repaired photomask with improved optical quality.

2. Discussion of the Prior Art

In the current state-of-the-technology, a number of publications areknown which disclose and teach the application of equipment and methods,which are required in order to remove defects encountered inlithographic masks. To that effect, an ulstrashort pulsed laser beam maybe utilized to ablate undesired extra material in a programmed spatialpattern, thereby removing the encountered defects. The foregoing aspectsare disclosed in Grenon, et al., U.S. Pat. Nos. 6,190,836; 6,165,649;6,156,461; 6,090,507; and Haight, et al., U.S. Pat. No. 6,333,485.

Furthermore, as known, laser ablation can produce a periodic structurein the irradiated material, thereby resulting in a significant degree ofroughness at the edge of the ablated defect, which degrades the opticalquality of the repaired photomask. This aspect is discussed in variouspublications, such as, for instance, the following articles: “LaserInduced Periodic Surface Structure: Experiments on Ge, Si, Al, andBrass”, Young, Preston, vsn Driel, and Sipe, Physical Review B, Vol. 27,No. 2, pgs. 1155-1172 (1983); “Ultraviolet Laser Induced PeriodicSurface Structures”, Clark and Emmony, Physical Review B, Vol. 40, No.4, pgs 2031-2041 (1989); “Femtosecond Laser Induced Periodic SurfaceStructure on Diamond Film”, Wu, Ma, Fang, Liao, Yu, Chen, Wang, AppliedPhysics Letters, Vol. 82, No. 11, pgs 1703-1705 (2003); and “SelfOrganixed Nanogratings in Glass Irradiated by Ultrashort Light Pulses”,Shimotsuma, Kazansky, Qui, Hirao, Physical Review Letters, Vol. 19, No.24, pgs 247205-1 to 4 (2003).

SUMMARY OF THE INVENTION

Ablation of materials using a femtosecond laser beam produces a finescale periodic structure in the ablated region. The structure consistsof residual (i.e. unablated material) and is always perpendicular to thepolarization direction of the laser beam. By changing the polarizationdirection during the ablation process, the structure is averaged overmany directions and thus eliminated.

Accordingly, it is an object of the present invention to provide amethod of eliminating structure and edge roughness in a material causedby the laser ablation of the material.

Another object of the invention resides in imparting the method asdescribed in an application to the repairing of photomasks so as tocause the optical quality thereof to be improved.

Yet another object is to provide a system of eliminating structure andedge roughness imparted to a material, such as a photomask, during laserablation of the material.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more readily apparent from the followingdescription and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. Also, the drawings are notnecessarily to scale, emphasis instead generally being placed uponillustrating the principles of the invention; wherein:

FIG. 1 is an illustrative picture of a repaired photomask in which adefect was removed by femtosecond laser ablation;

FIG. 2 shows illustrative pictures of periodic structures in ablatedlines as a function of the direction of polarization of the femtosecondlaser beam;

FIG. 3 is an illustrative block diagram of a system for rotating thepolarization of a laser beam to average the direction of the ablationstructure, and thereby eliminate it, according to one embodiment of theinvention; and

FIG. 4 is an illustrative picture of a line of ablated material in whichthe edge roughness (ablation structure) has been eliminated, accordingto one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Defects are commonly encountered during the fabrication of a photomask,whereby these defects generally consist of extra (unwanted) materialthat must be removed in order to create a perfect photomask. Femtosecondlasers can be used to ablate this extra material, thus removing thedefect. Hereby, the placement and spatial sharpness of the edge of therepaired region is critical to producing a high quality photomask.Anything that detracts from the placement and spatial sharpness of therepaired edge must be avoided.

Referring to FIG. 1, there is represented a picture of a repairedphotomask 10 in which a defect was removed by femtosecond laser ablationof a repair site 12. Laser ablation typically produces a highly periodicstructure 14, which degrades the optical quality of the repaired edge16. This periodic structure 14 can be traced to the polarization of thelaser beam, which is used to ablate the material.

Referring to FIG. 2, there are illustrated images of periodic structures20, 22, 24, 26, 28 and 30 formed in an ablated line as a function of thedirection of polarization 32, 34 and 36 of the femtosecond laser beam.These periodic structures 20, 22, 24, 26, 28 and 30 are always orientedperpendicular to the polarization directions 32, 34 and 36 of the laserbeam. If multiple polarization directions are used during the ablationsequence, the resulting structure consists of an average over thesedirections of the periodic structure formed by any individualpolarization direction. Thus, by performing the ablation using a seriesof polarization directions, the periodic structure is minimized oreliminated by means of averaging.

Referring to FIG. 3, there is shown a block diagram of a system forrotating the polarization of a laser beam to average the direction ofthe ablation structure, and thereby eliminated. The system includes a ¼waveplate 40, a focusing lens 42, and a photomask 44. Linearly polarizedlight 46, which is pulsed from a femtolaser 48 passes through the ¼waveplate 40 and is turned into circularly (or eliptically) polarizedlight 50. The circularly polarized light 50 passes through the focusinglens 42 and is incident on the photomask 44. In this case, thepolarization direction of the incident laser light would continuouslychange direction during each laser pulse. By way of example, for 100femtosecond, 266 nm laser pulses, which are employed for mask repair,the polarization direction would rotate a full 360 degrees throughapproximately one hundred times, thus averaging the periodic structureover all directions many times.

Since ablation occurs only over the portion of each laser pulse in whichthe laser amplitude exceeds the threshold for ablation, the effectivenumber of polarization direction cycles will be considerably less thanone hundred. At the limit which only the peak of the laser pulse ablatesmaterial (a situation which results in the highest spatial resolution),the effective polarization direction would be nearly identical for eachlaser if the amplitude of each laser pulse was nearly identical. This isundesirable since it reduces the amount of averaging over eachpolarization direction. Therefore, it is also advantageous if there issome pulse to pulse variation in the amplitude of laser pulses, and ifmultiple laser pulses overlap spatially. This variation will help torandomize the polarization directions from one laser pulse to the next.

Referring to FIG. 4, there is illustrated an image of a line of ablatedmaterial 60 in which the edge roughness (ablation structure) has beeneliminated. In this case, a quarter wave plate was inserted into thelaser path just prior to the laser beam entering the final focusinglens. There was approximately a 5% pulse to pulse variation in theamplitude of each laser pulse. The resulting ablation does not evidenceany of periodic structure, and thus the edges of the ablated region arevery smooth.

An alternative method of averaging over many polarization directionsinvolves a rotating half wave plate. By mechanically rotating a halfwave plate during an ablation, the polarization direction also rotates,thus averaging the periodic ablation structure. For example, therepaired region could be scanned repeatedly with the half wave platerotated by 90 degrees between each scan. This would produce an averageof two periodic structures oriented at 90 degrees to each other.

Another method of averaging over many polarization directions involvesinserting a Pockell Cell in the path of the laser beam. By applying avoltage to the Pockell Cell, the polarization direction can be rotatedto any desired angle. If the applied voltage is varied as the laser beamis scanned, averaging over any desired number of polarization directionscan be achieved.

Variations, modifications, and other implementations of what isdescribed herein may occur to those of ordinary skill in the art withoutdeparting from the spirit and scope of the invention. Accordingly, theinvention is not to be defined only by the preceding illustrativedescription.

1. A system of reducing periodic structures formed in a photomaskmaterial during a laser ablation process, the method system: a laser forproducing incident linearly polarized laser light; means for convertingthe incident linearly polarized laser light to circularly or elipticallypolarized laser light; and means for averaging a plurality ofpolarization directions during the laser ablation process.
 2. The systemas claimed in claim 1, wherein the means for polarizing and averagingcomprise a quarter wave plate inserted into an optical path of thelinearly polarized laser light to produce said circularly or elipticallypolarized light.
 3. The system as claimed in claim 1, wherein the meansfor polarizing comprises a half wave plate inserted into an optical pathof the linearly polarized laser light to alter the direction ofpolarization.
 4. The system as claimed in claim 1, wherein the means foraveraging comprises means for rotating the half wave plate during thelaser ablation process.